1. Field of the Invention
The present invention relates to an engine start control device for a vehicle having an idling-stop function.
2. Description of the Related Art
An idling-stop vehicle stops fuel supply to an engine when predetermined idling-stop conditions are satisfied. Therefore, after an engine rotation speed is reduced, the idling-stop vehicle comes into an engine stall state. A mechanical oil pump, which rotates in synchronization with the engine, discharges a reduced amount when the engine rotation speed is reduced. As a result, a hydraulic pressure for bringing a forward clutch (hereinafter, referred to simply as “clutch”) into engagement (hereinafter, referred to as “line pressure”) is reduced. Thus, even when control is performed so that the clutch maintains the engaged state, the hydraulic pressure is not supplied to an automatic transmission and the clutch in the engine stall state. As a result, the clutch cannot maintain the engaged state any more. If the engine is restarted in the above-mentioned state, the line the pressure (hydraulic pressure of a hydraulic circuit) and the engagement/disengagement of the clutch cannot be controlled. Therefore, the clutch is suddenly brought into the engaged state with a sudden increase in line pressure. Accordingly, there is a problem of shock generation.
In order to prevent the shock generation, there is a control method of releasing the clutch during idling-stop, restarting the engine in a clutch-released state, and slowly bringing the clutch into the engaged state after complete combustion. According to the above-mentioned method, however, the vehicle can be only started after a predetermined time period from the complete combustion in the engine to the full engagement of the clutch after the complete combustion. Even when the above-mentioned time period in which the vehicle cannot be started is reduced, a feeling of discomfort of a driver cannot be eliminated.
In view of the problem described above, it is considered to reduce the time until the vehicle starts by starting the engine while the engaged state of the clutch is maintained. As described above, however, the hydraulic pressure is not supplied from the mechanical oil pump during the idling stop, and hence it is necessary to supply the hydraulic pressure by another means.
As a method of solving the problems described above, there has been proposed a method involving connecting an electric oil pump in parallel to the mechanical oil pump and driving the electric oil pump during the idling stop to supply the hydraulic pressure so as to maintain the engaged state of the clutch.
According to the above-mentioned method, it is considered as follows. If the system is configured to drive the electric oil pump during a time period in which the driving of the mechanical oil pump is stopped, the hydraulic pressure, which allows the engaged state to be maintained, can be ensured. In this manner, a driving time can be minimized to further reduce the amount of electric power consumption. However, the following is found. There is a time lag from the start of the driving of the electric oil pump to the sufficient increase of the line pressure. As a result, within a specific period of time after the satisfaction of idling stop conditions, the line pressure is disadvantageously reduced to such a degree that the engaged state of the clutch cannot be maintained. If an engine restart request is made in the above-mentioned specific period of time, the shock is generated.
Therefore, there exists a conventional engine start control device which has a predetermined time period in which the restart is inhibited after the start of driving of the electric oil pump in case of an insufficient line pressure for the engagement of the clutch at the time of restart (for example, Japanese Patent No. 4566083 (page 9, FIG. 5)).
There exists another conventional engine start control device which includes a hydraulic pressure sensor. After it is detected by the hydraulic pressure sensor that the line pressure is insufficient, the driving of the electric oil pump is started. The driving of the electric oil pump is controlled so as to maintain the hydraulic pressure necessary for the engagement of the clutch (for example, Japanese Patent No. 4576713 (page 18, FIG. 5)).
The conventional start control device described in Japanese Patent No. 4566083 cited above is configured so that the electric oil pump is driven after the mechanical oil pump is stopped and the restart is inhibited until the hydraulic pressure sufficient for the engagement of the clutch is ensured. According to the method described above, a predetermined time is required after the issuance of the restart request to enable the actual vehicle start as in the case of the conventional device described above. Therefore, there is a problem in that the driver experiences a feeling of discomfort.
The start control device, which has a configuration using the hydraulic pressure sensor to detect a reduction in hydraulic pressure so as to drive the electric oil pump, is proposed in Japanese Patent No. 4576713 cited above. If the configuration described above is employed while using the electric oil pump which has a high responsiveness and a large dischargeable amount, the reduction in line pressure can be reduced. Ability to supply the hydraulic pressure which can maintain the engagement of the clutch in a steady state is sufficient as general performance required for the electric oil pump. In addition, cost can be reduced in such a case. However, if the start control device is configured by using the electric oil pump having the minimal performance as described above, a certain length of time is required to increase the hydraulic pressure to a predetermined pressure by the electric oil pump. Therefore, there is a problem in that the hydraulic pressure for continuously maintaining the engagement of the clutch cannot be ensured.
It is then considered that unexpected release of the clutch due to a reduction in hydraulic pressure can be prevented by driving the electric oil pump at earlier timing even if the electric oil pump having the minimal performance is used. In the case where the electric oil pump having the minimal performance is used, however, the reduction in line pressure cannot be sufficiently prevented even when the driving of the electric oil pump is started at earlier timing. Moreover, a check valve is generally connected to an output side of the electric oil pump for the purpose of preventing a backflow from the mechanical oil pump. Thus, if the driving of the electric oil pump is started in a state in which the line pressure is high to such a degree that the check valve is not opened, a pressure in the electric oil pump is abnormally increased to result in a failure.